1. Field of the Invention
This patent application claims the benefit of priority from Italian Patent Application No. MI12002A 001996 filed Sep. 19, 2002 through PCT Application Ser. No. PCT/IT2003/000523 filed Aug. 28, 2003, the contents of which are incorporated herein by reference.
The present invention relates to a process, as well as the corresponding production line, for manufacturing ultrathin hot strip, being rolled through a thermo-mechanical means to thicknesses down to a minimum of 0.4 mm based on the thin slab technology.
2. Description of Related Art
It is known that the so-called “thin slab” technique for manufacturing hot rolled strip has been strongly developing from the time of the first plants of this type in USA and Italy starting since years 1990 and 1992.
At present with this thin slab technology can already be produced as hot strip whichever quality of steel, both in the field of carbon steels and in that of stainless steel. The state of the art is described by way of example in DE 3840812C2, EP 0415987B1, DE 19520832A1 and WO 00/20141. Under a more attentive examination it appears that a hardly controllable parameter is the temperature: at a casting speed of 4-6 m/min and hot strip thickness <2 mm temperatures of the intermediate strip <900° C. (AC3) are measured at exit from the roughing mill and strip temperatures <750° C. (AC1) at exit from the finishing mill, which cause quality inconveniences as to the properties of the material and the production safety.
In order to avoid going below these critical temperatures, the thickness of the intermediate strip after the roughing or high reduction mill HRM at casting speeds of 4-6 m/min cannot be less than 20 mm. This value of the intermediate strip thickness leads e.g., after passing through the induction heating zone and reaching a strip temperature of about 1200° C. at the furnace exit, again to limits of the hot finished strip thickness, limits that it is impossible to exceed downwards without also reaching at the same time temperatures lower than AC1 temperature of 750° C., such as in case of a carbon steel with 0.06% C, with consequent drawbacks in the steel quality.
It is also known after ten years of productive experience and developments of the thin slab technique, that the trade demand has to be met with a hot rolled strip product of better quality and at lower costs. The requirements of the market for a hot rolled strip are directed in particular to a minimum thickness of 0.4 mm and at the same time a thermo-mechanical rolling in the meaning of the T.T.T. diagram, leading to the desired and improved mechanical characteristics of the material. In this context a low-cost production of Dual Phase, TRIP and TWIP steels production has been taken into consideration, in the best technical way by means of the thin slab technique.